The present invention relates to hydrostatic transmission apparatus for a vehicle having at least two drive members disposed one after the other in the direction of travel of said vehicle, the apparatus comprising at least two hydraulic motors suitable for driving respective ones of said drive members, each hydraulic motor having two main orifices serving respectively as feed and as discharge in a preferred direction of travel of the vehicle, a main hydraulic pump connected to two main ducts that serve respectively as feed and as discharge in the preferred direction, and a bypass valve suitable for taking up a reduced-speed position in which the feed main orifices of the motors are connected to the feed main duct, while the discharge main orifices are connected to the discharge main duct, and an increased-speed position in which at least one motor is bypassed, the feed main orifice of said motor being connected to the discharge main duct via a bypass link, while the other main orifices continue to have their respective links.
In this type of transmission, the hydraulic motors are connected in parallel to the main hydraulic pump. For operating the vehicle at reduced speed, e.g. in a working situation, such parallel connection makes it possible, as is known, to distribute the fluid between the motors as a function of their respective needs.
In order to operate the vehicle at increased speed, e.g. for traveling on the road, at least one of the motors is bypassed by the bypass link, since its feed main orifice is connected to the discharge main duct, as is the discharge main orifice of said motor.
It should be noted that the invention applies both to apparatus in which all of the motors are of single cylinder capacity, and also to apparatus in which at least one motor has a plurality of cylinder capacities that can be selected by a selector specific to the motor in question. Under such circumstances, there exists at least one situation in which the speed is higher than the speed of the reduced-speed and increased-speed situations described above. For example, a motor that is not bypassed when the bypass valve is in the increased-speed position can have two cylinder capacities. Therefore, even when the bypass valve is in its increased-speed position, there exists a situation in which the total cylinder capacity of the non-bypassed motor is active, thereby corresponding to a medium-speed situation, and another situation in which a reduced cylinder capacity of the non-bypassed motor is active, thereby corresponding to a high-speed situation.
In addition, the invention applies to apparatus having at least two motors for respectively feeding front and rear drive members of a vehicle. The vehicle may be a vehicle having two wheels, two rollers, or two sets of wheels, each of which is driven by a single motor, or indeed, for example, a vehicle having three or four wheels, each of which is driven by a respective motor.
It is known that such a vehicle can be braked, at least partially, hydrostatically, by a build-up of pressure in the main duct that serves as the discharge.
The hydrostatic braking effect is limited to the motor or to the group of motors whose main orifices are put at different pressures, by being connected to respective ones of the feed and of the discharge main ducts.
During hydrostatic braking, the pressure increases at the main orifice that is connected to the discharge. However, when the bypass valve is in its increased-speed position, both of the main orifices of the bypassed motor are connected to the discharge main duct, so that the hydrostatic braking has no effect on that motor.
Thus, only the braking torque offered by the non-bypassed motor(s) is available for slowing down the vehicle. Therefore the effectiveness of hydrostatic braking is limited.
An object of the invention is to remedy the above-mentioned drawbacks by proposing hydrostatic transmission apparatus that makes better hydrostatic braking possible, even in the increased-speed position.
This object is achieved by the fact that the apparatus of the invention further comprises constriction means suitable for being activated to restrict the flow of fluid through said bypass link.
Thus, during hydrostatic braking that is performed while the bypass valve is in its increased-speed position, even the bypassed motor(s) participate in the hydrostatic braking because the restriction in the flow of fluid in the bypass link puts the main orifices of each such bypassed motor at different pressures. In other words, the restraining torque is not only the restraining torque that is developed by the motor(s) that is/are not bypassed, but rather it is also the restraining torque that is developed by the bypassed motor and the restraining torque that is obtained by the constriction of the bypass link.
The effectiveness of hydrostatic braking is thus increased due to the restriction in the flow of fluid through the bypass link.
Advantageously, the bypass link extends between the discharge main duct and the bypass valve.
Under these conditions, the bypass valve can be situated at a node between the feed main orifice of the motor that can be bypassed, a link duct for linking with the feed main duct, and the bypass link. Depending on its position, the bypass valve can connect the main orifice of said motor to the link with the feed main duct, or to the bypass link.
Advantageously, the constriction means comprise a constriction valve suitable, in a free configuration, for enabling fluid to flow substantially freely in said bypass link when the fluid pressure in said link is less than a pressure threshold, and, in a constricted configuration, for restricting the flow of fluid in said link when said pressure is greater than said pressure threshold. The constriction valve is advantageously disposed in the bypass link.
When in its free configuration, the constriction valve makes it possible for the transmission to operate normally at increased speed, while at least one motor is bypassed, whereas said constriction valve can be placed in its constricted configuration when hydrostatic braking is necessary at a level such that the fluid pressure in the bypass link exceeds the pressure threshold.
This pressure threshold may be defined as a function of the additional hydrostatic braking torque necessary on the transmission of the vehicle. Advantageously, said threshold is at least equal to the boost pressure of the hydraulic circuit. For example, it is approximately in the range 10% to 20% of the maximum pressure of the circuit (which is, in general, defined by the pressure ratings of pressure limiters connected to the main ducts).
Advantageously, the apparatus further comprises a check valve that is disposed in parallel with the constriction valve so as to allow free flow in the bypass link, in the non-preferred direction of travel. It is also advantageous for the apparatus to further comprise means for detecting the direction of travel of the vehicle and for preventing the constriction valve from going into said constricted configuration when the non-preferred direction of travel is detected.
In a variant, advantageously, the apparatus further comprises a constriction bypass valve disposed in parallel with the constriction valve and suitable for taking up a constriction bypassed position allowing free flow in the bypass link, and a constriction active position in which the flow of fluid in the bypass link goes via the constriction valve.
Advantageously, the apparatus further comprises an electronic control unit that is suitable for acting on the basis of information relating to the conditions of travel of the vehicle either to allow the constriction valve to go into said constricted configuration or to prevent it from going into said constricted configuration.
These possibilities are particularly useful when it is desired, in certain operating situations, to sustain a high fluid flow rate in the bypass link, without causing the constriction valve to go into its position in which it restricts the flow through said link.
This applies particularly while the vehicle is travelling in its non-preferred direction (in reverse). The connection configuration of the check valve then makes it possible to obtain the desired fluid flow rate in reverse, with automatic operation that is controlled hydraulically.
When the information relating to the conditions of travel of the vehicle concerns the direction of travel of said vehicle and when the electronic control unit is associated with the means for detecting the direction of travel of the vehicle, a fluid flow is allowed in the bypass link while the vehicle is traveling in reverse.
However, there exist multiple vehicle travel conditions in which the electronic control unit can authorize or allow the constriction valve to go into its constricted configuration. For example, it might be desired to enable increased hydrostatic braking to take place under particular vehicle load conditions, or under particular conditions regarding distribution of load on the axles, and/or under particular travel conditions, whereas such a possibility might not be desirable in other situations.
Depending on the number and the placement of the drive members of the vehicle, the apparatus may further comprise at least one additional motor that, in the reduced-speed position, has its feed and discharge main orifices connected respectively to the feed and discharge main ducts and that, in the increased-speed position, has its feed main orifice connected to the discharge main duct via an additional bypass link, whereas its discharge main orifice is connected to the discharge main duct, and the apparatus further comprises additional constriction means suitable for being activated to restrict the flow of fluid in said additional bypass link.
In a variant, in the increased-speed position, the additional motor may be bypassed by having its feed main orifice connected to the discharge main duct via the bypass link, which link is common to said bypassed additional motor and to said at least one bypassed motor, whereas the discharge main orifices of said bypassed motors are connected to the discharge main duct, and the constriction means are suitable for being activated to restrict the flow of fluid in said common bypass link.
The invention can be well understood and its advantages appear more clearly on reading the following detailed description of embodiments shown by way of non-limiting example.